Leaving it open and letting people sign-up for guest accounts on your WordPress blog could lead to incredibly nasty stuff happening if anybody so desired. And trust me I am not exaggerating this. So don’t wait a second to disable this option and please relay the message.

WordPress dev team has been notified a while back and I dare hope they will soon start acting on it, if only by relaying a similar announcement through the official channel (as well as, of course, releasing a proper patch).